NOYES AND COOLIDGE, — ELECTRICAL CONDUCTIVITY. 175 



pressiire of 300 atmospheres has been used, the steel shell having previ- 

 ously been similarly tested up to 600 atmospheres pressure. The lining 

 must be fitted as closely as possible before the hydraulic pressure is 

 applied, siuce otherwise this will always result in tearing the lining. 

 Even after expanding the lining with hydraulic pressure, there is no 

 trouble in removing it, in case a leak develops, by the first method given 

 above. 



The next step is to fasten the edge of the flange to the shell. If this 

 is not done, when substances like benzophenone, solid at ordinary tem- 

 perature, are employed for heating the 

 bomb, they will be drawn under the 

 flange and into the groove in the shell, 

 where they will solidify ; upon heating 

 the bomb the next time, the solid melts 

 and escapes, thus relieving the pressure 

 on the packing ring and allowing the 

 bomb to leak : moreover, if the edge 

 is not fastened down, there is danger 

 of bending it when the bomb is opened 

 and handled. To secure the flange 

 eight small steel screws are used. The 

 steel shell has to be recessed at this 

 place, as shown in Fig. 1 ; otherwise Figure 5. 



the screw heads would interfere with the cover. 



The lower electrode is made of two steel parts, as shown in Fig. 5, 

 the horizontal part, C, being afterwards enclosed in a platinum box, which 

 is made as follows : The top, A, of this box is made by forcing a circular 

 disk of pure sheet platinum (about 0.25 mm. thick) through a brass die 

 by means of a brass punch. It is better, since it strains the platinum 

 less, to interrupt this operation at least twice, annealing the metal each 

 time. This box should be made to fit so tightly over C that it has to 

 be forced on. In the same way a tight fitting bottom, B, is made for 

 this box. A hole is drilled in the centre of this just large enough to 

 permit the passage of the steel rod through it. It is then forced on over 

 A. It then remains only to solder B to A with pure gold. This is 

 easily accomplished by putting several pieces of gold on the crack, D, 

 and directing a hot flame from the blast lamp downwards onto the box. 

 This flame must not be too small, since the whole of the soldering must 

 be done at once and as quickly as possible. Doing it a piece at a time 

 involves keeping the steel rod hot for a longer time, and consequently 



